If you’ve successfully flushed a toilet recently, then you
appreciate (at least subconsciously) the workings of a good sewer
system. Waste disappears, no matter what time of day or night, or
what the weather’s like.

But weather is actually a challenge for a sewer system,
especially precipitation. That New York City’s sewers can handle
the waste of more than 8 million residents on a daily basis —
plus the occasional downpour or major storm like Hurricane Irene
— is a testament to its resilience.

How does New York do it? Simple: combined sewer outflows. When
there’s simply too much water in the sewers for the city’s
wastewater treatment plants to cope, the proverbial flood gates
are opened and rainfall mixed with
sewage flows into area waterways, such as Newtown Creek
or the Gowanus Canal.

That’s great for keeping sewage from finding other places to go —
like back up into your toilet. But it’s less good for places that
consistently see such overflows, like the Gowanus Canal. The
canal, affectionately called “Lavender Lake” by locals for the
multi-hued sheen of its
near constant water pollution, carries a raging case of
gonorrhea thanks to all that outflow as well as some of the
most toxic sludge in the country. In fact, an industrial legacy
paired with these “combined sewer outflows” has been enough to
turn
the canal into a Superfund site — or one of the nation’s most
polluted localities.

That isn’t exactly sustainable. Especially once you consider that
downpours and the like are predicted to be on the increase as a
result of climate change, as are little threats like
sea level rise that could turn outflows into inflows.

In fact, though resilience and sustainability — two of the
hottest buzzwords in urban planning — are practically
used interchangeably, they are in fact in some tension with
each other. A resilient system bounces back from challenges,
unharmed, and a big part of building in resilience includes
building in ways to fail safely, such as the combined sewer
outflows. So, for example, the
blackout of 2003 showed how the U.S. power grid remains less
than resilient to challenges like untrimmed trees and power lines
sagging in the heat. An example of a more resilient technological
system is the Internet here in America, where if one route for
data fails, another is found.

Sustainability, on the other hand, means efficiency, at least in
part, as designers strive to strike a balance between human needs
and environmental impacts. This century, the world’s megacities
will swell to become
gigalopolises — vast tracts of urbanized land, like the
metropolitan corridor between Boston and Washington, D.C., or the
predicted one between Hangzhou and Shenyang. Efficiency in the
construction of infrastructure will be vital as the world
attempts to build in the next few decades the same amount of
urban infrastructure we’ve cobbled together over the last several
millennia. Does that mean foregoing a built-in margin of safety?
The Internet may be resilient in the United States, but a
reliance on single lines of connection to the rest of the world
has disconnected countries across Africa, from Egypt to Uganda.
Imagine the same thing happening to an “efficient” sewer system.

Some of the most obvious ways to become more resilient are not
sustainable. For example, if you are concerned about reliable
electricity, you can increase the resilience of your local grid
by buying a diesel generator, or two, or more. In effect, that’s
what the Googles, Facebooks, and Twitters of the world do. But
extra diesel generators are certainly not an efficient, or
particularly sustainable, way to create electricity. It’s not
ideal for the environment to be burning all that extra diesel,
with attendant air pollution and the like.

Fortunately, there are ways to think more creatively about how to
achieve the same resilience. So, instead of buying a couple big
extra generators, a thousand small generators on rooftops might
be better, as in the case of solar power on homes. Or programs
that reduce the overall demand for power — reducing the stress on
the existing electrical grid — can help avoid issues like the
2003 blackout.

Some 5 billion people are
projected to live in urban areas by 2030. These cities of the
future — most of them cities of today, like New York — will have
to cope with climate change, sea level rise, increasing demand
for electricity and the logistics of 5 billion peoples’ sewage,
among other things. New York, at least, has been working on a
plan to cope with climate change since 2007, thanks to subtle
reminders like an
August thunderstorm that year, which shut down the city’s
subway system during morning rush hour. Climate change is
predicted to increase the severity of such downpours. That’s just
as true for heartland cities like Louisville, which has seen
four weather-related disasters in as many years, spurring the
development of a climate change adaptation plan, and even
up-and-coming cities around the world, such as Surat in India,
which is
using money from the Rockefeller Foundation to better plan
for a history and future of flooding.

As for those combined sewer outflows, well, a little greenery
goes a long way.
Roofs planted with vegetation, streets that end in rain
gardens, even permeable pavement can all go a long way towards
making the New York City sewer system both more resilient and
more sustainable. In the end, any enduring city must be resilient
to a whole host of challenges — from floods and fire to economic
collapse — in order to last for the long-term. And what could be
more sustainable than that?